Sheets, tubes and other configurations of plastic materials are finding increasing use in a wide variety of applications today. The advent of low cost plastics has increased their availability as substitutes for traditional construction materials. Effective and economical reinforcing of these plastics to increase their strength while maintaining a cost advantage is still a problem.
A variety of methods have been used to encapsulate or otherwise combine a reinforcing material with a plastic material to form a laminate structure. For example, adhesives or heat can be used to combine two or more pieces of plastic material to encapsulate the reinforcing material. Several limitations and problems are present with such techniques. For example, the plastic is not one-piece or a continuous plastic material. Discontinuities in the plastic material may lead to delamination and degradation of the entire structure. An imperfect or partial encapsulation exposes the reinforcing material to potentially harmful environment, i.e. moisture, corrosives.
The manufacture of the laminate structure by known techniques can be complex. For example, the plastic material may be formed in a separate operation from the reinforcing material, necessitating additional manufacturing operations leading to quality control considerations and additional cost.
Another potential problem with known encapsulation methods is the lack of bonding or fusion between the reinforcing material and the plastic material. When adhesives are used, the strength of the structure is limited to the strength of the bonds between either the reinforcing material and the adhesive, the plastic material and the adhesive, or the adhesive itself. The weakest of these bonds limits the strength of the reinforced plastic material and thus its ability to substitute for traditional construction materials.
In particular, when using a fibrous reinforcing material, the fibers can appear on the surface of the laminate. This cosmetically detracts from the finish and increases the brittleness of the product. Also, the fibers are exposed to the environment and can absorb moisture, potentially causing structural failure or degradation of the laminate.
One method of forming a plastic laminate is disclosed in Japanese Patent Publication No. 37-7627 published July 9, 1962 from Patent Application No. 35-22059 filed Apr. 25, 1960 by Komatsu Chemical Products Company, Ltd. of Tokyo, Japan. This publication discloses a composite synthetic resin sheet that includes a laminate of surface synthetic resin layers and a middle layer of reinforcing sheet material such as wire netting and glass fabric. The process includes extruding a tubular synthetic resin material from a single extrusion machine through an annular die, cooling the resin to allow cutting, cutting the tubular resin material into two separate strips and flattening each of the strips by means of flat guide plates. Thereafter, the reinforcing sheeting material is fed between the two strips to a row press for combining them into a composite sheet. The resin material, after cutting, must be re-heated to complete the encapsulation of the reinforcing material. Since the reinforcing material is not encapsulated by a continuous structure, the slit area is subject to delamination.
Another method of forming a reinforced plastic sheet material is disclosed in U.S. Pat. No. 3,687,764. This patent discloses a web composed of uniformly arranged strands applied to the inner layer of a first thermoplastic sheet. A second thermoplastic sheet has its inner layer applied and contacting the first plastic sheet through the holes provided in the web. The web of reinforcing material may be made of woven or non-woven strands arranged in an open mesh network and may comprise inorganic material such as metals, glass and asbestos. Although the multilayer thermoplastic film avoids the necessity of applying adhesives, one of the layers of thermoplastic material must have a lower sealing temperature than the other thermoplastic material, resulting in a discontinuous structure of two different thermoplastic materials. A potential problem disclosed by the patent is the lack of bonding between the reinforcing material and the thermoplastic materials which weakens the laminate.